As the cooling system for a hot-rolled steel strip, there are ones disclosed, for example, in Patent Literatures 1 to 3. Specifically, Patent Literature 1 describes a system including a first cooling apparatus arranged immediately after a last stand of a finishing mill line of hot rolling equipment. The first cooling apparatus includes: nozzles for forming a band-like or oblong jet impingement area on a surface, which is to be cooled, of a steel plate; and a damming roll for damming cooling water jetted from the nozzles. In this system, a pool of the cooling water is formed in an area between a roll of the last stand and the damming roll, and the damming roll is arranged such that the steel plate, transported through the first cooling apparatus, is immersed in the cooling water of the pool.
Further, Patent Literature 2 describes an online cooling system including multiple cooling units arranged in a conveyance direction. Each of the cooling units includes: multiple rotating rolls for pressing a thick steel plate from above and below; cooling water headers arranged above and below the thick steel plate; and a large number of nozzles provided in the cooling water headers. On the delivery side of each cooling unit, liquid jet nozzles are arranged at predetermined intervals in a longitudinal direction of each cooling water header extending in a strip-widthwise direction, and slit nozzles for air jetting are arranged in the vicinities of the liquid jet nozzles, so as to prevent cooling water from diffusing on the delivery side of each of the cooling units.
Further, Patent Literature 3 describes an apparatus where a pair of work rolls, which come into contact with a rolled plate, are constituted as extremely-small diameter rolls or different-diameter rolls. This apparatus is provided with: a roll cooling device for jetting a spray of water to a surface of each of the work rolls; and a plate cooling device for jetting a spray of water from a surface of the steel plate on the delivery side of the work rolls toward a contact point between the steel plate and the work rolls. This apparatus is also provided with a water cutoff device which is brought into contact with the surfaces of the work rolls or is separated from the surfaces thereof, thereby shutting off or opening the passage of the spray water to the surface of the steel plate.
A hot-rolled steel strip (steel plate) made of fine-grained steel is well known to have excellent mechanical properties, such as strength and toughness, from Patent Literature 3 mentioned above and Patent Literature 4 mentioned later, and the like. This provides such effects as reducing the weight of a device or an apparatus formed of the hot-rolled steel strip and reducing consumption energy by the weight reduction, and therefore has drawn attention from the industry.
Then, Patent Literature 3 states “Patent Literature 1 (referred to as Patent Literature 5 in this document) discloses a rolling mill for manufacturing a hot-rolled steel plate (steel strip) made of fine-grained steel in hot rolling by a so-called high reduction rolling method where a rolled plate is subjected to intensive cooling while being subjected to rolling at a high reduction rate (high reduction) during hot rolling.” That is, structural refinement is achieved by high reduction, and a rolled plate that generates working heat according to high reduction is kept in a suitable temperature range (around an Ar3 transformation point) by intensive cooling, and grain growth is thereby stopped, and a fine-grained steel hot-rolled steel plate is thus obtained.
Further, Patent Literature 4 states that a hot-rolled steel strip made of a fine-grained structure, where a grain size of a ferrite structure is 3 to 4 μm or less for example, is obtained by: performing hot rolling on a steel containing 0.3% by weight or less of C and 3% by weight or less of alloy elements other than C in the process of cooling the steel from a temperature range of the Ar3 transformation point or more; in the final stage, applying hot rolling on the steel, with a total surface reduction rate of 50% or more to 95% or less, once or more than once substantially within one second in a temperature range of (Ar1+50° C.) to (Ar3+100° C.); and performing cooling to a temperature range of 600° C. or less at a cooling rate of 20° C./s or more to 2000° C./s or less after the hot rolling.